a) Field of the Invention
The invention is directed to an apparatus for the analysis of retinal vessels which contains a retinal camera for recording images of the ocular fundus, at least one image storage, means for detecting arterial and venous vessels in the images, means for determining the vessel diameters of the detected arterial and venous vessels, and means for determining artery-to-vein ratios (AVRs) from the determined vessel diameters.
b) Description of the Related Art
The invention is further directed to a method for the analysis of retinal vessels through the evaluation of digital fundus images in which a ring-shaped measurement zone containing vessel portions which proceed from a central vessel, which are to be distinguished as arteries or veins, and which are to be measured is arranged around the papilla at a distance therefrom, a vessel diameter being determined for each of these vessel portions, wherein a retinal arterial vessel diameter equivalent and a retinal venous vessel diameter equivalent are determined from the determined vessel diameters of the arterial and venous vessel portions, and an artery-to-vein ratio is determined from the vessel diameter equivalents of the arteries and veins.
By means of a quantitative vessel analysis using digital fundus images, it is possible to assess vascular risk which can be determined, e.g., by means of an artery-to-vein ratio (AVR) according to Hubbard et al. (Ophthalmology, Vol. 106, December 1999, pages 2269-2280). For example, in a study (ARIC study) it was possible to determine a relationship between the artery-to-vein ratio and the cardiovascular risk in women and for stroke and diabetes in women and men independent from blood pressure and other risk factors.
Wong T Y et al. (Retinal microvascular abnormalities and incident stroke: The Atherosclerosis Risk in Communities Studies, The Lancet, Vol. 358 (2001), pages 1134-1140) show that risk groups for stroke and other severe vascular diseases can be classified based on the AVR for estimating a systemic microvascular risk.
All of the systems for determining the artery-to-vein ratio have a retinal camera, units for storing the recorded individual image, means for detecting arterial and venous vessels, means for determining vessel diameters, and means for calculating the AVR.
A digital fundus image recorded by the retinal camera is evaluated such that diameters of arteries and veins located within a ring-shaped measurement zone around the papilla are determined according to a standardized protocol and summarized according to a standardized model formula to form a retinal arterial vessel diameter equivalent CAE and a retinal venous vessel diameter equivalent CVE whose quotient (CAE/CVE) gives a value for the AVR. Different model formulas may be applied (Knudsen M D et al., Revised formulas for summarizing retinal vessel diameters, Current Eye Research, Vol. 27 (2003) 143-149).
A procedure of this kind is suitable only for epidemiological studies with a high number of cases (10,000 or more) because systematic error proportions of the individual AVR values are substantially randomized and along with the random errors of the individual AVR values only play a subordinate role for epidemiological studies and evaluations in group averages.
However, the methods are very uncertain and are usable only conditionally for individual diagnoses because, as a result of the random systematic errors which vary between individuals, the scatter of the individual central equivalents and of the artery-to-vein ratios in individual examinations is very high and leads to unsatisfactory reproducibility. Under certain conditions, this scatter can spread over a plurality of risk assessment groups so that the risk classification of a patient can vary between high risk and normal risk in studies that are repeated after short intervals. Therefore, the static vessel analysis can only be used conditionally.